Downhole logging tools are normally lowered into boreholes or cased wellbores to make formation evaluation measurements and infer properties of the formation surrounding the borehole and the fluids (gas, oil, water, or a mixed multi-phase) in the formation. These downhole tools may be an acoustic/ultrasonic logging tool, a neutron porosity tool, gamma-ray tool, density tool, a formation identification tool for measuring the earth formations surrounding a borehole, such as in a hydrocarbon (e.g., oil, natural gas, etc.) well. A wireline cable not only mechanically supports the downhole tool but also simultaneously provides electrical power to the tool and sends the measured data back to a surface data process unit. Such downhole logging instruments may be used to make such measurements while the well is being drilled, which is referred to as logging-while-drilling (LWD) or measurement-while-drilling (MWD). A downhole logging tool has electrical conductors mounted on the tool housing in a tubular structure. The logging tool includes a metal housing and with at least an electrical feedthrough or a plurality of electrical feedthroughs. An electrical feedthrough is coupled to the wireline cable from exterior downhole tool enclosure, and to the interior electronic circuits, and the internal sealing material is used to insulate the electrical transmissions from logging tool electronics to the wireline cable.
Electrical feedthroughs used in wireline tools, logging while drilling (LWD) and measurement while drilling (MWD) tools, as well as any other electronic instruments capable of being used in a downhole environment are subjected to a variety of harsh operating environments, which may exhibit pressures up to 30,000 PSI and temperatures up to 300 degrees Celsius. An electrical feedthrough consists of metal pin(s), sealed in an insulating material that may carry substantial amounts of power, either a few thousand volts or a few hundred ampere electric currents. The electronics inside the downhole tool requires a hermetic type electrical feedthrough that interconnects with surface instruments either for power and control signal transmission or for data transmission. The electrical feedthroughs must survive in such extreme hostile liquid environments such as brine, oil and water base drilling mud and fluids that may contain hydrogen sulfide, carbon dioxide, methane, and moisture.
Most of the used sealing materials are based on thermoplastic materials (such as Viton, Teflon, PEEK, and glass-reinforced-epoxy, etc.) for insulating the electrical transmissions from logging tool electronics to the wireline cable but the mechanical creeps under elevated temperature often limits its maximum continuous operating temperature to less than 200 degrees Celsius because of their low glass transition temperature (for example, Tg(PEEK)=143 degrees Celsius). Alternatively, a glass-to-metal dielectric sealing could provide high sealing strength, toughness and high resistivity and break-down voltage but its hermetic seal is frequently breakdown by high mechanical stress, introduced either by installation deformation or by mismatched coefficients of thermal expansion in the package materials. To improve glass-to-metal seal based high-pressure and high-temperature (HPHT) downhole electrical feedthrough reliability there is a need for providing a robust electrical feedthrough package and seal method.
For thermoplastic sealed electrical feedthroughs the thermoplastic material may be subject to creep and delamination between the metal pins and thermoplastic material, resulting in moisture passage into downhole logging tool electrical circuit sections. The glass-to-metal sealed electrical feedthrough and interconnector have the high mechanical strength and insulation strength that could protect inside logging or measurement electronics or instruments from extreme hostile liquid environments, but the seal may fail because of issues in potential high compressive stress in sealing material, installation induced mechanical deformation, low volumetric resistivity at elevated temperature, conductive ion fluid condensation, or/and high moisture absorption etc. Specifically, if the coefficient of thermal expansion (CTE) mismatch between the metal shell and the sealing material induced compressive stress has exceeded the maximum allowable strength of the sealing material, the micro or macro cracks could lead to the loss of the hermeticity and also to the water absorption for deteriorating the insulation strength. It is desirable that an electrical feedthrough package can be easily installed with downhole tool and bulkhead but also have highly reliable mechanical seal and electrical seal for operating from −40 degrees Celsius to 300 degrees Celsius with pressure up to 30,000 PSI.
Therefore a need exists for novel electrical feedthroughs for use in high-pressure, high-temperature, and other harsh environmental applications for providing downhole logging tools, LWD and MWD tools, power, signal, and data transmission. A further need exists for novel electrical feedthrough packages which may be able to prevent catastrophic downhole tool electric failures. There is also a need for novel electrical feedthrough packages which are able to operate for extended periods in water-based or moisture-rich oil-based wellbores. Furthermore, a need exists for novel electrical feedthrough packages which are able to operate while being repeatedly exposed to harsh environments such as found in open holes and closed bore holes.